Expansion Valve Position Control Systems And Methods

ABSTRACT

A system includes a load module, a comparison module and a control module. The load module is configured to determine a current load of a compressor. The comparison module is configured to compare the current load to a previous load of the compressor to generate a comparison signal based on the comparison. The control module is configured to generate (i) a first control signal based on a superheat value of the compressor, and (ii) a second control signal based on the current load and the previous load. The control module is configured to, based on the comparison signal, control a position of an expansion valve according to either the first control signal or the second control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional application No. 61/729,029, filed on Nov. 21, 2012. The entire disclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates to cooling systems, and more particularly, expansion valve control systems.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

Cooling systems have applicability in a number of different applications where a fluid is to be cooled. The fluid may be a gas, such as air, or a liquid, such as water. Example applications are heating, ventilation, air conditioning (HVAC) systems that are used for cooling spaces where people are present such as offices and data center climate control systems. A data center may refer to a room having a collection of electronic equipment, such as computer servers.

In FIG. 1, an air conditioner 50 that may be used in, for example, a computer room is shown. The air conditioner 50 includes a cooling circuit 51 and a cabinet 52. The cooling circuit 51 is disposed in the cabinet 52 and includes an evaporator 54, an air moving device 56, a compressor 58, a condenser 60, and an expansion valve 62. The evaporator 54, compressor 58, condenser 60 and expansion valve 62 are connected in a closed loop in which a cooling fluid (e.g., phase change refrigerant) circulates. The evaporator 54 may include a V or A-coil assembly with multiple cooling slabs to provide increased cooling capacity. The evaporator 54 receives the cooling fluid and cools air passing through openings in evaporator 54. The air moving device 56 (e.g., a fan or squirrel cage blower) draws the air from an inlet (not shown) in the cabinet 52 and through the evaporator 54. The cooled air is directed from the evaporator 54 and out a plenum 64 in the cabinet 52.

The compressor 58 circulates the cooling fluid through the condenser 60, the expansion valve 62, the evaporator 54 and back to the compressor 58. The compressor 58 may be, for example, a scroll compressor. A scroll compressor may be a fixed speed, digital speed, or variable speed compressor. A scroll compressor typically includes two offset spiral disks. The first spiral disk is a stationary disk or scroll. The second spiral disk is an orbiting scroll. The cooling fluid is received at an inlet of the scroll compressor, trapped between the offset spiral disks, compressed, and discharged at a center (or outlet) towards the condenser 60. The condenser 60 may be a micro-channel condenser that cools the cooling fluid received from the compressor 58. The expansion valve 62 may be an electronic expansion valve and expand the cooling fluid out of the condenser 60 from, for example, a liquid to a vapor.

A position of the expansion valve 62 (or opening percentage of the expansion valve) may be adjusted to control a suction superheat value of the compressor 58. The superheat value of the compressor is equal to a compressor suction temperature minus a compressor saturated suction temperature. A compressor suction pressure may be used to determine the compressor saturated suction temperature. The compressor suction temperature and the compressor suction pressure may be determined based on signals from corresponding sensors connected between the evaporator 54 and the compressor 58. The superheat value refers to an amount that a temperature of a cooling fluid, in a gas state, is heated above the compressor saturated suction temperature.

The superheat value can be used to modulate (or adjust) position of the expansion valve 62. Position (or opening percentage) control of the expansion valve 62 may be performed by a proportional, integral, derivative (PID) control module. The PID control module controls the superheat value to match a constant predetermined superheat setpoint. This ensures compressor reliability and improves compressor efficiency.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

In an aspect, a system is provided and includes a load module, a comparison module and a control module. The load module is configured to determine a current load of a compressor. The comparison module is configured to compare the current load to a previous load of the compressor to generate a comparison signal based on the comparison. The control module is configured to generate (i) a first control signal based on a superheat value of the compressor, and (ii) a second control signal based on the current load and the previous load. The control module is configured to, based on the comparison signal, control a position of an expansion valve according to either the first control signal or the second control signal.

In another aspect, a system is provided and includes a load module and a control module. The load module is configured to detect a change in a load of a compressor. The control module is configured to generate (i) a proportional, integral, derivative (PID) control signal based on a superheat value of the compressor, and (ii) an expansion valve control signal based on the change in the load. The control module is configured to, in response to the change in the load, transition from controlling a position of an expansion valve according to the PID control signal to controlling the position of the expansion valve according to the expansion valve control signal.

In another aspect, a method is provided and includes determining a current load of a compressor and comparing the current load to a previous load of the compressor. A comparison signal is generated based on the comparison. Based on the comparison signal, the method further includes either: generating a first control signal based on a superheat value of the compressor and controlling a position of an expansion valve according to the first control signal; or generating a second control signal based on the current load and the previous load and controlling the position of the expansion valve according to the second control signal.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1, is a perspective view of a prior art air conditioner;

FIG. 2, is a schematic view of a multi-stage cooling system incorporating a cooling control module in accordance with an aspect of the present disclosure;

FIG. 3, is a functional block diagram of a superheat regulation system in accordance with an aspect of the present disclosure; and

FIG. 4 is a logic flow diagram illustrating a superheat regulation method in accordance with an aspect of the present disclosure.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example implementations will now be described more fully with reference to the accompanying drawings.

An electronic expansion valve (EEV) of an air conditioning system and/or cooling circuit may be controlled by a PID control module. The PID control module may control (sometimes referred to as modulate) a position (or open percentage) of the EEV based on changes in a superheat value of a compressor.

The PID control module has an associated delay when responding to changes in the superheat value. This delay does not cause an issue when the compressor is operating at a constant load or load that changes less than a predetermined amount over a predetermined period of time. The PID control module may not respond quick enough during compressor mechanical loading and unloading changes that are greater than the predetermined amount. This can occur, for example, when operating a digital scroll compressor or a semi-hermetic compressor with cylinder unloaders.

Due to the delayed response of the PID control module, the PID control module may overshoot a superheat setpoint thereby providing either a high superheat overshoot condition or a low superheat overshoot condition. In addition, the delayed response may also provide a low compressor ratio (or pressure differential). A pressure differential of a compressor refers to a difference between an inlet or suction pressure and an outlet or head pressure of the compressor. A low pressure differential (or differential less than a predetermined differential) can result in damage to the compressor, compressor failure, and/or unstable compressor and/or system operation.

The implementations disclosed herein provide EEV control techniques that include quick superheat adjustments during compressor loading and unloading conditions. The implementations include use of multiple superheat control methods to prevent (i) high and low superheat overshoot conditions from arising and (ii) occurrences of low compressor ratios.

In FIG. 2, a schematic of a cooling system 100 is shown. The cooling system 100 includes an upstream cooling stage 102 with an upstream (or first) cooling circuit 104 and a downstream (or second) cooling stage 106 with a downstream cooling circuit 108. The cooling circuits 104, 108 are controlled via a cooling control module 109. Although two cooling circuits are shown, a different number of cooling circuits may be included. The upstream cooling circuit 104 includes a first evaporator 110, a first expansion valve 112, a first condenser 114, a first compressor 116, and a second compressor 118. The downstream cooling circuit 108 includes a second evaporator 120, a second expansion valve 122, a second condenser 124, a third compressor 126, and a fourth compressor 128. The evaporators 110, 120 have respective evaporator fans 130, 132 or in serial to have common fans. The condensers 114, 124 have respective condenser fans 134, 136 or in serial to have common fans.

The cooling control module 109 may generate condenser fan signals COND1, COND2, evaporator fan signals EVAP1, EVAP2, expansion valve signals EXP1, EXP2, and compressor signals PWM1, PWM2, PWM3, PWM4 to control the fans 130, 132, 134, 136, expansion valves 112, 122, and the compressors 116, 118, 126, 128.

The cooling control module 109 may control the fans 130, 132, 134, 136, the expansion valves 112, 122, and/or the compressors 116, 118, 126, 128 based on signals from various sensors. The sensors may include, for example, an ambient temperature sensor 150, suction pressure sensors 152, 154, head pressure sensors 156, 158 and/or compressor inlet (or evaporator outlet) temperature sensors 160, 162. The ambient temperature sensor 150 may be an outdoor ambient temperature sensor and generate an ambient temperature signal T_(A). The pressure sensors 152, 154 generate suction pressure signals SUC1, SUC2 and detect pressures of fluid received by the compressors 116, 118, 126, 128. The head pressure sensors 156, 158 generate head pressure (or discharge pressure) signals HEAD1, HEAD2 and detect pressures of fluid out of the compressors 116, 118, 126, 128. The temperature sensors 160, 162 detect temperatures of fluids (i) downstream from the evaporators 110, 120, and (ii) between the evaporators 110, 120 and the compressors 116, 118, 126, 128.

The evaporators 110, 120 may be micro-channel (MC) cooling coil assemblies and/or includes a MC heat exchanger and/or may be fin-and-tube cooling coil assemblies. The expansion valves 112, 122 are electronic based expansion valves (e.g., EEVs). Each of the condensers 114, 124 may be of a variety of types, such as an air-cooled condenser, a water-cooled condenser, or glycol cooled condenser. The condensers 114, 124 may include heat rejection devices that transfer heat from return fluids to a cooler medium, such as outside air. The heat rejection devices may include air or liquid cooled heat exchangers.

In each of the cooling circuits 104, 108, a cooling fluid (or refrigerant) is circulated by a respective pair of the compressors 116, 118, 126, 128. The fluids flow from the compressors 116, 118, 126, 128, through the condensers 114, 124, expansion valves 112, 122, and evaporators 110, 120 and back to the compressors 116, 118, 126, 128. The evaporators 110, 120 are arranged in stages such that air flows in a serial fashion first through the upstream evaporator 110 and then through the downstream evaporator 120. By having multiple cooling stages arranged for serial air flow, a temperature differential across the evaporators 110, 120 is reduced. This in turn allows the evaporators 110, 120 to operate at different pressure levels and allows the pressure differences between the respective evaporators 110, 120 and condensers 114, 124 to be reduced.

Since compressor power is a function of a pressure difference between an evaporator and a condenser, a lower pressure difference is more energy efficient. Each of the cooling circuits 104, 108 may include a pair of tandem compressors (e.g., compressors 116, 118 or compressors 126, 128). Each of the tandem compressors may be a fixed capacity scroll compressor (e.g., compressors 116, 126) or a variable capacity scroll compressor (e.g., compressors 118, 128). The fixed capacity scroll compressors may be activated (powered ON) and deactivated (powered OFF) based on control signals generated by the cooling control module 109. The variable capacity scroll compressors may be controlled via a respective digital signal received from the cooling control module 109.

Each of the cooling circuits 104, 108 may include a tandem set of compressors. Each of the tandem sets may include two compressors of equal volumetric displacement. The first compressor may be a digital pulse width modulation (PWM) scroll compressor that receives a PWM percentage signal to control speed and capacity of the first compressor. The second compressor may be a fixed speed scroll compressor with simply ON/OFF capacity control. Suction and discharge lines of these two compressors may be piped in parallel to form the tandem set. As an example, compressors 116, 126 may be PWM scroll compressors and compressors 118, 128 may be fixed speed scroll compressors. The fixed speed scroll compressors may receive ON/OFF control signals rather than PWM signals from the cooling control module 109.

The tandem set compressor configuration allows for energy efficient temperature control by providing a wide range of capacity modulation for a cooling circuit of an air conditioning system. The tandem sets offer an energy efficient configuration at compressor startup by allowing the digital PWM scroll compressors to be activated prior to the fixed speed scroll compressors. This effectively allows the tandem sets to provide partial-displacement operation with a reduced volumetric displacement/capacity until additional capacity is needed from the fixed scroll compressors.

As used herein, a compressor pressure differential refers to a difference between input and output pressures of the compressor. A low-pressure differential (less than a predetermined differential and/or suction and head pressures of the compressor are equal to each other) can cause an unloaded compressor condition. Compressor unloading can lead to reduced cooling capacity of the compressors of a tandem set at startup and potential damage to the tandem set and/or associated compressor motors. Unloading of the compressors reduces the ability of the tandem set to move vapor, which reduces cooling capacity. This reduction in the pressure differential can also cause damage to compressor motor(s) if the reduction occurs repeatedly.

Referring also to FIG. 3, a functional block diagram of a superheat regulation system 200 is shown. The superheat regulation system 200 includes the cooling control module 109 and a cooling circuit 202 (e.g., one of the cooling circuits 104, 108). The cooling control module 109 includes a superheat module 204, a first summer 206, a PID control module 208, a compressor load module 210, a position determination module 212, and an expansion valve (EV) control module 214.

The superheat module 204 receives sensor signals from sensors 216 (e.g., sensors 154, 156, 160, 162) of the cooling circuit 202 and/or a saturation temperature SatTemp from a saturation module 218. The sensor signals may include a suction pressure signal SucPres and a compressor inlet temperature signal CompINTemp. The saturation module 218 determines the saturation temperature SatTemp of a compressor (e.g., one of the compressors 116, 118, 126, 128) of the cooling circuit 202 based on the suction pressure signal SucPres. The superheat module 204 may include a second summer 220, which may subtract the saturation temperature SatTemp from the compressor inlet temperature CompINTemp to generate a superheat signal SH. The super heat signal SH may include a current superheat value indicating a superheat condition of the compressor.

The first summer 206 subtracts the superheat signal SH from a superheat setpoint SET to generate an error signal ERROR. The superheat setpoint SET may be indicated by a setpoint module 222. The setpoint module 222 generates the superheat setpoint SET, which may be a predetermined value.

The PID control module 208 provides proportional, integral, derivative control of a position of an EV of the cooling circuit 202. The PID control module 208 generates a control signal PIDCONT (or first control signal) to control the position of the EV based on the error signal ERROR. The PID control module 208 may have tuning parameters such as proportional, integral and derivative gains, which may be used to determine PID values for EV control.

The compressor load module 210 determines a load on the compressor (or compressor load) and generates a load signal LOAD indicating the determined load. A compressor load may refer to a percentage level of a total available compressor load. The total available compressor load may refer to total possible loading of one or more compressor(s). A compressor load may refer to total amount of loading available for a tandem set of compressors. A compressor load may be directly proportional to a duty cycle (or an ON time versus OFF time) of a compressor.

As an example, in one implementation a tandem set with two compressors is used and includes a digital scroll compressor and a fixed scroll compressor. The digital scroll compressor may be used when compressor loading is less than or equal to, for example, 50% of a total available compressor load. The fixed scroll compressor may be used in addition to the digital scroll compressor when compressor loading is greater than 50%. If the compressor loading is 60%, 50% compressor load may be provided by the fixed scroll compressor and 10% compressor load may be provided by the digital scroll compressor. Loading of the digital scroll compressor may be ramped down when the fixed scroll compressor is turned ON and may be ramped up when the fixed scroll compressor is turned OFF.

The load signal LOAD may be generated based on cooling requests and/or dehumidification requests to adjust a temperature and/or a humidity level within a room or predetermined area. The cooling requests and/or dehumidification requests may be generated by the cooling control module 109 in response to temperature and humidity level inputs (collectively input signals INPUT) to the cooling control module 109. The temperature and humidity level inputs may be provided via, for example, a user interface 224, which may be connected to or included as part of the cooling control module 109. The user interface 224 may include a keypad, a display, a touchscreen, or other suitable interface.

The position determination module 212 determines a position or opening percentage of an EV 226 (e.g., one of the EVs 112, 122) and generates a second control signal OPEN %. The position determination module 212 generates the second control signal OPEN % based on the load signal LOAD. The second control signal OPEN % may be generated according to, for example, equation 1, where OPEN %_(Prev) is a previously determined opening percentage or first value of the second control signal OPEN %, OPEN %_(Cur) is a current opening percentage or second value of the second control signal OPEN %, CompLoad_(Cur) is current compressor load or current value of the load signal LOAD, CompLoad_(Prev) is previous compressor load or previous value of the load signal LOAD, a is first input constant, and b is a second input constant (or exponent constant). The variables a and b may be predetermined values.

$\begin{matrix} {{{OPEN}\mspace{14mu} \%_{Cur}} = {{aOPEN}\mspace{14mu} {\%_{Prev}\left\lbrack \frac{{CompLoad}_{Cur}}{{CompLoad}_{Prev}} \right\rbrack}^{b}}} & (1) \end{matrix}$

The compressor load module 210 and the position determination module 212 may be used to anticipate a change in a compressor load and/or a superheat value and predict the amount of that change. A large step change in compressor load (e.g., change due to turning ON or OFF a compressor or a change greater than a predetermined threshold) may be predicted based on the cooling and dehumidification requests. To account for a reaction time of a fluid in the cooling circuit 202 to change due to change in requested load, the position determination module 212 generates the second control signal OPEN % based on a predicted change in compressor load to satisfy the cooling and dehumidification requests.

The second control signal OPEN % may be generated, where a current compressor load may be CompLoad_(Prev) and an estimated or predicted compressor load may be CompLoad_(Cur). The second control signal OPEN % may be generated prior to the change in the compressor load to the estimated or predicted compressor load. The EV control module 214 may generate an EV control signal EV based on the second control signal OPEN % while the compressor load change is occurring and/or within a predetermined period of the change in the compressor load.

The EV control module 214 generates the EV control signal to adjust the position of the EV based on the control signals PIDCONT and OPEN %. The EV control module 214 includes a comparison module 230 and an evaluation module 232. The comparison module 230 may receive the load signal, a current compressor load LOAD_(C), and/or a previous compressor load LOAD_(P). The compressor loads LOAD_(C), LOAD_(P) may be determined based on the load signal LOAD. The comparison module 230 may also receive a predetermined threshold PREDTHR and a predetermined time TIME_(PRED) from a memory 235. The memory 235 stores predetermined thresholds and times 236. The comparison module 230 determines whether a difference between the loads LOAD_(C), LOAD_(P) is greater than the predetermined threshold PREDTHR for the predetermined time TIME_(PRED). The comparison module 230 may include a timer module 234 to compare a current value of a timer 238 to the predetermined time TIME_(PRED). The comparison module 230 generates a comparison signal LC based on the comparison.

The evaluation module 232 receives the control signals PIDCONT, OPEN % and generates an EV control signal EV based on the control signals PIDCONT, OPEN %. In one implementation, the evaluation module 232 determines whether to generate the EV control signal based on the first control signal PIDCONT or the second control signal OPEN %. This determination is based on the comparison signal LC.

The superheat regulation system 200 may be operated using numerous methods, an example method is provided by the method of FIG. 4. In FIG. 4, a logic flow diagram illustrating a superheat regulation method is shown. The method may begin at 300 and at 306 and may be performed by the cooling control module 109. Although the following tasks are primarily described with respect to the implementations of FIGS. 2-3, the tasks may be easily modified to apply to other implementations of the present disclosure. The tasks may be iteratively performed and are directed to two algorithms and/or control methods, which may be performed in parallel. The first method, referred to as the PID control method, starts at 300 and includes tasks 300-304. The second method, referred to as the compressor load based method, starts at 306 and includes tasks 302 and 306-316.

At 302, the evaluation module 232 determines whether a request to change EV position has been received and/or generated due to a current or anticipated change in compressor load. This request may refer to the comparison signal LC, which indicates whether a change in compressor load is greater than the predetermined threshold PREDTHR. The evaluation module 232 performs task 304 when the compressor load is constant, is to remain constant, and/or the change in compressor load is less than or equal to the predetermined threshold PREDTHR. The change in compressor load may be a current detected change or a predicted change. Task 316 is performed when the request is received, the request is generated, and/or the change in compressor load is greater than the predetermined threshold PREDTHR.

At 304, the evaluation module 232 may control the EV 226 based on the first control signal PIDCONT and not based on the second control signal OPEN %. The position (or opening percentage) of the EV 226 is adjusted in response to the first control signal PIDCONT. Task 302 is performed subsequent to task 302.

At 308, the comparison module 230 reads or determines the first compressor load Load₁ and resets and starts the timer 238. The comparison module 230 may receive or determine the first compressor load Load₁ based on the load signal LOAD and store the first compressor load Load₁ in the memory 235. The timer module 234 resets and/or starts the timer 238.

At 310, the timer module 234 determines whether a value of the timer 238 is equal to the predetermined time TIME_(PRED). Task 310 provides a delay between tasks 308 and 310. When the change in compressor load is predicted, task 310 may not be performed. If the value of the timer 238 is equal to the predetermined time TIME_(PRED), task 312 is performed.

At 312, the comparison module 230 reads, determines, and/or predicts a second compressor load Load₂ and the position of the EV 226 and stops the timer 238. The second compressor load Load₂ may be determined and/or predicted based on cooling and dehumidification requests. The position of the EV 226 or the previous open percentage OPEN %_(PREV) may be determined based on a previous EV control signal and/or stored in the memory 235. The comparison module 230 may receive or determine the second compressor load Load₂ based on the load signal LOAD and store the second compressor load Load₂ in the memory 235.

At 314, the comparison module 230 may access the memory 235 to obtain the compressor loads LOAD₁, LOAD₂, compares the compressor loads LOAD₁, LOAD₂, and/or determine whether a change in compressor load is greater than the predetermined threshold PREDTHR. If the change in compressor load is greater than the predetermined threshold PREDTHR, then task 302 is performed, otherwise task 308 is performed. If task 302 is to be performed, the comparison module 230 may request that EV position control be performed using the compressor load based method and not the PID control method. If task 308 is to be performed, PID control of the position of the EV 226 is continued at 304.

Tasks 308-314 may be performed to predict a change in compressor load greater than the predetermined threshold PREDTHR. Tasks 308-314 may be performed to anticipate a large change in compressor load, such as when a fixed compressor of a tandem set is turned ON or turned OFF. Tasks 308-314 may be performed prior to the change in the compressor load from the first compressor load LOAD₁ to the second compressor load LOAD₂ actually occurring. The second compressor load LOAD₂ may be determined based on cooling and dehumidification requests, as described above. Task 316 may be performed while the actual change in compressor load from the first compressor load LOAD₁ to the second compressor load LOAD₂ occurs and/or within a predetermined period of the change in the compressor load.

At 316, the evaluation module 232 generates the EV control signal based the second control signal OPEN %, which is based on previous and current compressor loads and a position (or opening percentage) of the EV 226. The second control signal OPEN % may be calculated, for example, according to equation 1 provided above using the first and second loads LOAD₁, LOAD₂ respectively as the previous and current compressor loads CompLoad_(PREV), CompLoad_(Cur). The previous open percentage OPEN %_(PREV) of equation 1 may be determined based on a previous EV control signal.

In one implementation, the PID control method is used as the default control method. The position of the EV 226 may be adjusted once each time the change in compressor load is greater than the predetermined threshold PREDTHR, as determined at 314. Control may return to adjusting the position of the EV 226 based on PID control subsequent to the compressor load based change. As an example, the comparison signal LC may be equal to 1 or TRUE when the change in compressor load is greater than the predetermined threshold PREDTHR, and may be 0 or FALSE when the change in compressor load is less than or equal to the predetermined threshold PREDTHR. The compressor load based method may be used while the comparison signal LC is equal to 1 and the PID control method may be used when the comparison signal is equal to 0.

The above-described tasks are meant to be illustrative examples; the tasks may be performed sequentially, synchronously, simultaneously, continuously, during overlapping time periods or in a different order depending upon the application. Also, any of the tasks may not be performed or skipped depending on the implementation and/or sequence of events.

The foregoing description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that one or more steps within a method may be executed in different order (or concurrently) without altering the principles of the present disclosure.

As used herein, the term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC); an electronic circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor (shared, dedicated, or group) that executes code; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as in a system-on-chip. The term module may include memory (shared, dedicated, or group) that stores code executed by the processor.

The term code, as used above, may include software, firmware, and/or microcode, and may refer to programs, methods, functions, classes, and/or objects. The term shared, as used above, means that some or all code from multiple modules may be executed using a single (shared) processor. In addition, some or all code from multiple modules may be stored by a single (shared) memory. The term group, as used above, means that some or all code from a single module may be executed using a group of processors. In addition, some or all code from a single module may be stored using a group of memories.

The apparatuses and methods described herein may be implemented by one or more computer programs executed by one or more processors. The computer programs include processor-executable instructions that are stored on a non-transitory tangible computer readable medium. The computer programs may also include stored data. Non-limiting examples of the non-transitory tangible computer readable medium are nonvolatile memory, magnetic storage, and optical storage.

Although the terms first, second, third, etc. may be used herein to describe various elements, components and/or modules, these items should not be limited by these terms. These terms may be only used to distinguish one item from another item. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first item discussed herein could be termed a second item without departing from the teachings of the example implementations.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A system comprising: a load module configured to determine a current load of a compressor; a comparison module configured to compare the current load to a previous load of the compressor to generate a comparison signal based on the comparison; and a control module configured to generate (i) a first control signal based on a superheat value of the compressor, and (ii) a second control signal based on the current load and the previous load, wherein the control module is configured to, based on the comparison signal, control a position of an expansion valve according to either the first control signal or the second control signal.
 2. The system of claim 1, wherein the control module is configured to control the position of the expansion valve according to the second control signal in response to the comparison signal being greater than a predetermined threshold.
 3. The system of claim 2, wherein the control module is configured to control the position of the expansion valve according to the first control signal in response to the comparison signal being less than or equal to the predetermined threshold.
 4. The system of claim 2, further comprising a timer module configured to compare a value of a timer to a predetermined time, wherein the control module is configured to (i) determine the previous load and then start the timer, and (ii) determine the current load when the timer value is equal to the predetermined time.
 5. The system of claim 1, wherein the control module is configured to default to controlling the position of the expansion valve according to the first control signal and controls the position of the expansion valve according to the second control signal when the change in the load is greater than a predetermined threshold.
 6. The system of claim 1, wherein the first control signal is at least one of a proportional, integral, and derivative control signal.
 7. The system of claim 1, wherein the control module is configured to generate the first control signal based on a superheat setpoint, wherein the control module is configured to determine the superheat value based on (i) a suction temperature of the compressor, and (ii) a suction pressure of the compressor.
 8. The system of claim 1, wherein the control module is configured to generate the second control signal based on a ratio between the current load and the previous load.
 9. The system of claim 1, wherein the control module is configured to generate the second control signal based on a previous version of the second control signal.
 10. The system of claim 9, wherein the control module is configured to generate the second control signal based on a first input constant, a ratio of the current load and the previous load, and a second input constant.
 11. A system comprising: a load module configured to detect a change in a load of a compressor; and a control module configured to generate (i) a proportional, integral, derivative (PID) control signal based on a superheat value of the compressor, and (ii) an expansion valve control signal based on the change in the load, wherein the control module is configured to, in response to the change in the load, transition from controlling a position of an expansion valve according to the PID control signal to controlling the position of the expansion valve according to the expansion valve control signal.
 12. The system of claim 11, wherein the control module is configured to default to controlling the position of the expansion valve according to the PID control signal and controls the position of the expansion valve according to the expansion valve control signal when the change in the load is greater than a predetermined threshold.
 13. The system of claim 11, wherein the control module is configured to generate the expansion valve control signal based on a ratio between the load and a previous load of the compressor.
 14. The system of claim 11, wherein the control module is configured to generate the expansion valve control signal based on a previous version of the expansion valve control signal.
 15. The system of claim 14, wherein the control module is configured to generate the expansion valve control signal based on multiplication of (i) a first input constant, (ii) the previous version of the expansion valve control signal, and (iii) a ratio of the load and a previous load to an exponential input constant.
 16. A method comprising: determining a current load of a compressor; comparing the current load to a previous load of the compressor; generating a comparison signal based on the comparison; and based on the comparison signal, either generating a first control signal based on a superheat value of the compressor and controlling a position of an expansion valve according to the first control signal, or generating a second control signal based on the current load and the previous load and controlling the position of the expansion valve according to the second control signal.
 17. The method of claim 16, further comprising controlling the position of the expansion valve according to: the second control signal in response to the comparison signal being greater than a predetermined threshold; and the first control signal in response to the comparison signal being less than or equal to the predetermined threshold.
 18. The method of claim 16, further comprising: controlling the position of the expansion valve according to the first control signal and not the second control signal when the change in the load is less than or equal to a predetermined threshold; and controlling the position of the expansion valve according to the second control signal and not the first control signal when the change in the load is greater than the predetermined threshold.
 19. The method of claim 16, further comprising generating the second control signal based on: a ratio between the current load and the previous load of the compressor; and a previous version of the second control signal.
 20. The method of claim 16, further comprising: generating the first control signal based on a superheat setpoint; determining the superheat value based on (i) a suction temperature of the compressor, and (ii) a suction pressure of the compressor; and generating the second control signal based on a previous position of the expansion valve and a ratio of the current load and the previous load. 